I'm planning to make stacks of 12 boards. I've chosen a OCZ ZX850W PSU. It's a single rail with max. 70A at +12V. It has 4 PCI-e connectors, so I can connect to 4 boards. From each of this boards I'll daisy chain to 2 other boards by the terminal block. Power should be sufficient. The PSU should operate in its efficient range, and the current on the single wires is not too high.